1. Field of the Invention
The present invention relates to the field of medicine generally, and more specifically to microbiology, immunology, vaccines and the prevention of infection by a bacterial pathogen by immunization.
2. Summary of the Related Art
Neisseria meningitidis is a leading cause of bacterial meningitis and sepsis throughout the world. The incidence of endemic meningococcal disease during the last thirty years ranges from 1 to 5 per 100,000 in the developed world, and from 10 to 25 per 100,000 in developing countries (Reido, F. X., et al. 1995). During epidemics the incidence of meningococcal disease approaches 1000 per 1000,000. There are approximately 2,600 cases of bacterial meningitis per year in the United States, and on average 330,000 cases in developing countries. The case fatality rate ranges between 10 and 20%.
Pathogenic meningococci are enveloped by a polysaccharide capsule that is attached to the outer membrane surface of the organism. Thirteen different serogroups of meningococci have been identified on the basis of the immunological specificity of the capsular polysaccharide (Frasch, C. E., et. al. 1985). Of these thirteen serogroups, five cause the majority of meningococcal disease; these include serogroups A, B, C, W135, and Y. Serogroup A is responsible for most epidemic disease. Serogroups B, C, and Y cause the majority of endemic disease and localized outbreaks.
The human naso-oropharyngeal mucosa is the only known natural reservoir of Neisseria meningitidis. Colonization takes place both at the exterior surface of the mucosal cell and the subepithelial tissue of the nasopharynx. Carriage of meningococci can last for months. Spreading of meningococci occurs by direct contact or via air droplets. Meningococci become invasive by passing through the mucosal epithelium via phagocytic vacuoles as a result of endocytosis. Host defense of invasive meningococci is dependent upon complement-mediated bacteriolysis. The serum antibodies that are responsible for complement-mediated bacteriolysis are directed in large part against the outer capsular polysaccharide.
Vaccines based on meningococcal polysaccharide have been described which elicit an immune response against the capsular polysaccharide. These antibodies are capable of complement-mediated bacteriolysis of the serogroup specific meningococci. The meningococcal polysaccharide vaccines were shown to be efficacious in children and adults (Peltola, H., et al. 1977 and Artenstein, M. S., et. al. 1970), but the efficacy was limited in infants and young children (Reingold, A. L., et. al. 1985). Subsequent doses of the polysaccharide in younger populations elicited a weak or no booster response (Goldschneider, I., et. al. 1973 and Gold, R., et. al. 1977). The duration of protection elicited by the meningococcal polysaccharide vaccines is not long lasting, and has been estimated to be between 3 to 5 years in adults and children above four years of age (Brandt, B., et. al. 1975, Käyhty, H., et al. 1980, and Ceesay, S. J., et. al. 1993). For children from one to four years old the duration of protection is less than three years (Reingold, A. L., et. al. 1985).
Polysaccharides are incapable of binding to the major histocompatibility complex molecules, a prerequisite for antigen presentation to and stimulation of T-helper lymphocytes, i.e., they are T-cell independent antigens. Polysaccharides are able to stimulate B lymphocytes for antibody production without the help of T-helper lymphocytes. As a result of the T-independent stimulation of the B lymphocytes, there is a lack of memory induction following immunization by these antigens. The polysaccharide antigens are capable of eliciting very effective T-independent responses in adults, but these T-independent responses are weak in the immature immune system of infants and young children.
T-independent polysaccharide antigens can be converted to T-dependent antigens by covalent attachment of the polysaccharides to protein molecules (“carriers” or “carrier proteins”). B cells that bind the polysaccharide component of the conjugate vaccine can be activated by helper T cells specific for peptides that are a part of the conjugated carrier protein. The T-helper response to the carrier protein serves to augment the antibody production to the polysaccharide.
The serogroup B polysaccharide has been shown to be poorly to non-immunogenic in the human population (Wyle, F. A., et. al. 1972). Chemical attachment of this serogroup polysaccharide to proteins has not significantly altered the immune response in laboratory animals (Jennings, H. J., et. al. 1981). The reason for the lack of immune response to this serogroup polysaccharide is thought to arise from structural similarities between the serogroup B polysaccharide and polysialylated host glycoproteins, such as the neural cell adhesion molecules.
A meningococcal conjugate vaccine based on serogroup C polysaccharide has been described. This monovalent vaccine elicits a strong functional antibody response to the capsular polysaccharide present on strains of N. meningitidis corresponding to serogroup C. Such a vaccine is only capable of protecting against disease caused by serogroup C bacteria.
Existing vaccines based on meningococcal polysaccharide are of limited use in young children and do not provide long-lasting protection in adults. The only meningococcal vaccine which as been shown to be capable of eliciting long-lasting protection in all groups, including children, at risk for meningococcal infection is based on a polysaccharide from a single serogroup of N. meningitidis and provides no protection against infection by other serogroups. Thus, a need exists for a meningococcal conjugate vaccine capable of conferring broad, long-lived protection against meningococcal disease in children and adults at risk for meningococcal infection. The multivalent meningococcal polysaccharides of the present invention solve this need by providing vaccine formulations in which immunogenic polysaccharides from the major pathogenic serogroups of N. meningitidis have been converted to T-dependent antigens through conjugations to carrier proteins.